The TV network may be a cable TV network, a closed-circuit TV network or a community TV distribution network, e.g. in an appartment block. Such networks are conventionally provided with means for simultaneously transmitting different programs over different channels. Some of the programs may be received over the air and be applied to the network by a combination of receiver and frequency transposing equipment, while other programs may be derived from local equipment, e.g. video recorders, TV cameras, and other forms of TV signal generators that can be directly connected to cables.
Naturally it is important to have means for allocating channels to the various sources of the TV programs. Such channel-allocating means include modulators for transposing the sound and video signals to appropriate parts of the frequency spectrum. This process may include transposing the frequency of signals received on one channel for re-transmission on another.
It is convenient to refer to modulation equipment that receives IF input as a modulator, and modulation equipment that receives its HF input as a transposer. In either case, the modulation equipment requires synchronization provided by local oscillators.
It sometimes happens that interfering signals, either from internal or from external sources, are received at frequencies very close to the locally transmitted frequencies thereby interfering with said transmissions.
One known example is the interference suffered by the video synchronizing signals of one channel on the network due to signals at the same frequency F and derived from intermodulation between two frequencies within the equipment whose sum or difference happens to be equal to F.
Another known example is disturbance of the video synchronizing signals of one channel of the network due to video synchronizing signals transmitted outside the network but on the same channel within .DELTA.F. Such a signal may be channelled to the users of the network by the network itself. This leads to moire patterns in the pictures received by the networks users, and can also result in loss of frame or line hold.
It is not always possible to avoid such intermodulation, particularly when channel space is limited either in the cable network or on the airwaves. Further in many cases, there is no possibility of acting on the interfering transmitter which is outside the control of the network.